This invention relates to a suspension arrangement for a tilting chair of the type where a back and seat of the chair tilt rearwardly under pressure from an occupant with the back tilting through an angle greater than that of the seat.
The tilt mechanisms on chairs and particularly office chairs are commonly of three types. Firstly, there is a type in which the back of the chair tilts rearwardly under pressure from the occupant while the seat remains fixed. Secondly, there is a type in which the back and seat tilt together as a fixed unit. Thirdly, there is a type with which the present invention is concerned in which the back tilts at a larger angle than the seat.
The third type of tilting function is generally found on high quality and more expensive chairs and has advantages which are not provided by the more simple types mentioned previously.
A conventional device or suspension arrangement for providing a movement of this type is provided by a first pivot pin device which mounts a back support member of the chair on a base frame member so that the back can move rearwardly about a horizontal axis relative to the base frame. The base frame is generally attached to or includes legs by which the chair is mounted on the ground so the base frame remains stationary while the back tilts rearwardly under pressure from the occupant of the chair.
The seat is then mounted for pivotal movement about a second horizontal axis parallel to the first and forwardly of the first on the base frame so that it too can tilt rearwardly and downwardly under pressure from the occupant. In order to link the back and the seat for conjoint movement while controlling the angles of movement so that the back tilts more than the seat, the seat is attached to the back support by a third pivot pin arrangement.
The seat then tilts with the back but through an angle dependent upon the relative distances between the first and third pivot pins and between the second and third pivot pins. Generally, the ratio of the distances is chosen to be approximately 2:1 so that the back can tilt through, for example, 20.degree. while the seat pivots through 1O.degree..
An alternative arrangement for providing this tilting movement is shown in U.S. Pat. No. 3,072,436 (Moore). In this arrangement the seat and back are mounted for pivotal movement about pivot pins with the control of the movement of the seat being provided by sliding of a pin within a slot.
In both of these arrangements, the pivotal movement takes place about pivot pins with spring force being provided by a separate spring which resists the movement of the back and seat. The spring can in fact be positioned at any location in the mechanism and merely acts to apply a biassing force to maintain the seat and back in the upright or rest position.